Glazing materials sometimes include one or more functional layers engineered to enhance the performance of the glazing. One important functional layer reduces transmission of infrared radiation. Infrared-rejecting functional layers are typically made of partially transparent metallized or dyed polymer film constructions that reflect or absorb unwanted solar radiation. References describing such functional layers include U.S. Pat. Nos. 4,590,118, 4,639,069 and 4,799,745. Conductive functional layers employing metallized film constructions can also be used for purposes such as antennae, electromagnetic interference (EMI) shielding, and for electrically heated film applications such as de-fogging, de-misting, defrosting or deicing of glazing and displays. Electrically heated film applications can require substantial current-carrying ability, but must employ a very thin (and accordingly very fragile) metal or metal alloy layer when visible light transparency is required. References describing such electrically heated film applications include U.S. Pat. Nos. 3,529,074, 4,782,216, 4,786,783, 5,324,374 and 5,332,888. Electrically heated films are of particular interest in vehicular safety glazing. Conventional vehicular safety glazing is a laminate of two rigid layers, typically glass, and an anti-lacerative mechanical energy-absorbing interlayer, typically plasticized polyvinyl butyral (“PVB”). Electrically heated glazing is prepared by placing the PVB layer, an electrically heated film layer and suitable electrodes between the glass layers, eliminating air from the engaging surfaces, and then subjecting the assembly to elevated temperature and pressure in an autoclave to fusion bond the PVB, electrically heated film layer and glass into an optically clear structure. The resulting electrically heated safety glazing can be used, for example, in the windshield, backlight, sunroof or side windows of an automobile, airplane, train or other vehicle.
To enhance vehicle aerodynamics and improve outward visibility, many vehicular window shapes are not planar, and increasingly include severe angles and complex curves. These angles and curves can make it very difficult to laminate an electrically heated functional layer into vehicle glazing. Sometimes electrical or optical defects arise immediately after lamination, and sometimes failures take place during use of the glazing.